We have initiated a study of the regulation of gene expression in the animal-like eukaryote Tetrahymena. In preliminary investigations, we have found that the transcription of the galactokinase gene is inhibited by glucose, epinephrine and cyclic AMP; the inhibition effect of glucose and epinephrine is apparently the result of increased intracellular levels of cyclic AMP. We propose to investigate the transcriptional regulation of the galactokinase gene directly by performing mRNA-DNA hybridization studies with recombinant DNA probes. These probes will be obtained by transformation of galactokinase-deficient E. coli with Tetrahymena-plasmid recombinant DNA and subsequent isolation of cloned DNA from gal ion transformants. We will also investigate the mechanism by which glucose and epinephrine modulate cyclic AMP levels in order to effect the regulation of galactokinase gene expression. These studies will be carried out in parallel with wild-type cells and otherwise isogenic mutant strains which exhibit altered regulation of the galactokinase gene. Characterization of such mutants will further resolve the nature and role of the components involved in the cyclic AMP-mediate regulation of specific gene transcription by glucose and epinephrine in Tetrahymena. Exploitation of this model system through the studies we have proposed should contribute to an understanding of the mechanisms of hormone and cyclic nucleotide-controlled gene expression in higher cells.